222 FRAGMENTS OF SCIENCE 



flank Glen Spean. He followed it eastward. The bottom 

 of the Spean Valley, like the others, gradually rose, and 

 therefore gradually approached the road on the adjacent 

 mountain- side. He came to Loch Laggan, the surface of 

 which rose almost to the level of the road, and beyond 

 the head of this lake he found, as in the other two cases, 

 a col, or watershed, at Makul, of exactly the same level 

 as the single road in GHen Spean, which, it will be remem- 

 bered, is a continuation of the lowest road in Glen Roy. 



Here we have a series of facts of obvious significance 

 as regards the solution of this problem. The effort of the 

 mind to form a coherent image from such facts may be 

 compared with the effort of the eyes to cause the pictures 

 of a stereoscope to coalesce. For a time we exercise 

 a certain strain, the object remaining vague and indis- 

 tinct. Suddenly its various parts seem to run together, 

 the object starting forth in clear and definite relief. 

 Such, I take it, was the effect of his ponderings upon 

 the mind of Sir Thomas Dick-Lauder. His solution was 

 this: Taking all their features into account, he was con- 

 vinced that water only could have produced the terraces. 

 But how had the water been collected ? He saw clearly 

 that, supposing the mouth of Glen Gluoy to be stopped 

 by a barrier sufficiently high, if the waters from the 

 mountains flanking the glen were allowed to collect, they 

 would form behind the barrier a lake, the surface of which 

 would gradually rise until it reached the level of the col 

 at the head of the glen. The rising would then cease ; the 

 superfluous water of Glen Gluoy discharging itself over 

 the eol into Glen Roy. As long as the barrier stopping 

 the mouth of Glen Gluoy continued high enough, we 

 should have in that glen a lake at the precise level of 



